Figure 1.

Cumulative frequency distribution of the 20 standard amino acids in Archaea, Bacteria,
and Eukaryota. Normality tests with SAS statistic software showed that the frequencies of the 20
amino acids in the three domains followed a normal distribution (data not shown).
The cumulative frequency ranged from 0 to 1. The distribution followed an S- shaped
curve and the largest change in cumulative frequency occurred when the amino acid
frequency was highest. The null hypothesis was that there was no difference between
amino acid frequency in the proteomes and the frequency based on the universal genetic
codes, namely the number of synonymous codons divided by the total number of sense
codons. Difference analysis among the amino acid frequencies from different taxa was
performed at the 0.05 level using Duncan's multiple-range test. The data were displayed
as mean and variance. The superscripts a, b, c and d are Duncan groupings, which indicated
that amino acid frequencies within the same grouping were not significantly different.
The descending order of the amino acid frequencies correspond to the Duncan groupings
from a to d.